2001 dodge ram 5.9 overheating

When the engine is at temp, there are 2 hoses which come out of the top of the engine which should be hot.

The smaller one goes to the heater core (Hot to h.c. on diagram). The larger one goes to the radiator (hot to rad). I colored the thermostat in green.

At all times, the water pump is pumping coolant at a rate determined only by the engine RPMs. The rate does not change depending on whether the thermostat is open or closed. Coolant is also being sent through the heater core at all times.

When the engine is cold, the thermostat is closed, and the water pump is pumping (technically pulling) coolant through the heater core. Because the (small) heater core hoses are restrictive, excess coolant is sent through the bypass.

As the engine begins to warm, the Hot to h.c. hose will begin to warm up. If you do not have the heater turned on, the cold from h.c. hose will also be warm, because there heater core fan is not extracting heat from the coolant running through the heater core.

When the engine gets hot, 1 of 2 things will happen:

1. If the temperature outside is cold and you have the heater on in the cabin, the thermostat might not open, because the heater core may be providing all the cooling the engine needs. For example, if you are driving on the highway and the temperature is 15° F, hot coolant will be sent through the heater core, and the freezing air being blown over the heater core will cool the hot coolant. Combined with the cold air flowing around the engine, this might provide all the cooling the engine requires.

2. If the temperature outside is hot, the coolant inside the engine will get hot enough to open the thermostat (green). The water pump will then begin pumping hot coolant into the radiator, and "cold" coolant out of the radiator into the engine. Because the radiator hoses are quite large, the bypass is no longer needed. (Note: hot coolant will still be pumped into the heater core, and it will be coming out of the heater core hot.)

A few possibilities (when engine is HOT)

1. If the Hot to h.c. hose is not getting hot, you might have a clogged heater core. Sometimes rust gunk can build up inside the heater core. This is not a big deal, as the main purpose of the heater core is to provide the cabin heat. It cannot cause the engine to overheat.

2. If the hot to rad hose (aka upper radiator hose) is not getting hot, you can consider yourself "lucky", because the problem is almost certainly in the cooling system, and not the engine. Did you uses the correct temperature thermostat when you replaced the thermostat? They range from 160° to 195°. You could try putting a 160° thermostat in. This would cause the thermostat to open when the coolant reaches 160°, which would in theory prevent the engine from getting as hot. Also, try unplugging the hot to rad hose from the radiator. It should start shooting (very) hot coolant out. If it doesn't shoot hot coolant, you either have a bad water pump, or a clog in the cooling system.

3. If the cold from rad hose (aka lower radiator hose) is very warm or hot, your radiator is not cooling the coolant enough. This could be causes by a bad fan clutch

4. It could be as simple as having a bad temperature sensor. There are probably 2 temp sensors located near the thermostat. One sends info to the dashboard temp gauge, the other sends info to the ECU. Since they are pretty cheap, it wouldn't hurt to replace them both.

Firstly you need to determine where in the system you have heat and where you lose it. carefully feel the top and bottom of the radiator hose, whichever one is hot to the touch is the side that's closest to the engine, the coolest one is where you need to begin your troubleshooting. things to look out for. -proper installation of thermostat. -Clogged hoses, mainly heater core hoses are the trouble makers, due to them being so small, the smallest particles can become trapped and create a blockage. -Contaminated system. What i mean by contamination is the lack of maintenance in the past has deteriorated the system internally and now you have this rust colored fluid with chunks of sludge. This is the worst case scenario which has light at the end of the tunnel but requires patience and determination.